Clathrate Physical Chemistry
Clathrate hydrates are crystalline compounds defined by the inclusion of a guest molecule within a hydrogen bonded water lattice. Quantum physical forces such as van der Waals forces and hydrogen bonding are involved in creating and maintaining these clathrate hydrate structures. Gas hydrates are a subset of clathrate hydrates wherein the “guest” molecule is a gas at or near ambient temperatures and pressures. Such gasses include methane, propane, carbon dioxide, hydrogen and many others. Clathrate hydrates are defined by four primary physical characteristics that are of practical engineering interest: an ability to adsorb remarkably large amounts of guest molecules within a hydrogen bonded lattice; an ability to separate gas mixtures based on the preferential formation of one gas hydrate over another; a large latent heat of formation that is similar to that of ice, but dependent on the specific guest molecule and additives; and a formation temperature generally higher than that required to convert water to ice.
The unique physical characteristics of clathrate hydrates described above have led, over the last fifty years, to the proposed application of clathrate hydrates to a number of distinct industrial uses including gas transport, gas storage, thermal energy storage, desalination and gas separation.
Historically, most research has been devoted to prevention of clathrate hydrate formation in gas pipelines. Aside from such prevention focused art, the largest proportion of prior art is focused on devising methods for transportation of natural gas by ship using clathrate hydrates. Much of this prior art has focused on various strategies for continuous production of a gas hydrate. See, for example, U.S. Pat. Nos. 2,356,407, 6,082,234, 6,038,235, 6,180,843, 5,964,093, 6,082,118, 6,653,516, and 6,653,516. Proposed continuous production systems typically contemplate the manufacture of a clathrate hydrate ‘slurry’ (loosely aggregated or suspended in solution) by mixing a clathrate hydrate forming gas and water at low temperature and high pressure in a manner designed to maximize the surface contact area between the two. Many of these continuous production models contemplate complex, refinery-like processes that require additional large systems for movement and storage of the hydrate product. The requirement for large custom engineered machinery is likely to make these systems prohibitively expensive for moderate to small-scale storage or transport operations.
Some prior art has proposed strategies for production of hydrate in enclosed systems that also store the product for future use. These include U.S. Pat. Nos. 4,920,752, and 5,540,190. Of particular interest is the quiescent hydrate system using surfactants described in U.S. Pat. No. 6,389,820 to Rogers, which is incorporated by reference herein. Rogers demonstrated that the addition of a small amount of the appropriately chosen surfactant could increase the formation rate of gas hydrate by more than 700 times. A rapid formation rate for clathrate hydrate is important for commercial applications. Another advantage of the Rogers demonstration is that the resulting hydrate in this method has very little occluded water. Density of successfully formed clathrate hydrate is important for commercial applications. Further, the Rogers demonstration unit was simple with few moving parts. A simple clathrate hydrate mechanism that avoids complex multi-step processing is of great engineering and commercial interest. However, the system described by Rogers in DOE report DE-AC26-97FT33203 suffers from a number of deficiencies with regard to prospective commercial application. A serious deficiency that has not been addressed is the buildup of gas hydrate on the heat exchanger surfaces, which results in unacceptably low thermal conductivity. A further limitation of the Rogers system is the large amount of thermal energy required for clathrate hydrate formation that is lost upon dissociation of the clathrate hydrate in order to re-produce the guest molecule. Also, the Rogers demonstration system does not address important requirements for a system that directly meets varying commercial needs for cogeneration, cost, mobility, operations management, permanence, safety, scale, serviceability, and thermal preservation and reuse. Finally, the Rogers system is focused on a single surfactant whereas there are many surfactants in the class of surfactant used by Rogers and many more probable formation accelerators now being identified in emerging art.
As briefly detailed above, despite 50 or more years of effort, there have been many constraints to efficient, controlled formation of clathrate hydrates and, ultimately, to commercialization of systems based on clathrate hydrates. Many well-known academic researchers, research institutions, and corporate laboratories throughout the world are focused on further improving the gas clathrate formation process by speeding formation, lowering required pressure, increasing required temperature for hydrate formation. Recently, the pace of research has accelerated with promising results. Recent published and/or patented art has identified and defined new mechanisms and potential mechanisms by which formation of natural gas clathrates can be made significantly more efficient. Such art includes the use of certain formation catalysts such as surfactants, hydrotropes, H-hydrate promoters, and activated carbon, which increase the efficiency of clathrate hydrate formation as well as various approaches to increase the rate of thermal transfer.
Natural Gas Transport
Natural gas is currently transported from gas fields to end users via two primary methods: Gas Pipelines and Liquified Natural Gas (LNG). A more recent proposed development has been the use of Compressed Natural Gas (CNG) for gas transport (U.S. Pat. Nos. 6,584,781 and 5,803,005). Another recently applied method for transporting natural gas has been the conversion of the Gas to Liquids (GTL), such as Methanol, Dimethyl Ether (DME) and Fischer Tropsch Diesel (FT-Diesel). Each of these technologies, Gas Pipelines LNG, CNG and GTL, has advantages and disadvantages.
Discussion of Current Gas Transportation Systems
Gas pipelines transport the majority of the world's natural gas to end users. Pipelines can be the most economic method of transporting large volumes of gas over short to medium distances. However, pipelines as a universal means of moving natural gas are limited by geographic, logistical, political, and territorial constraints. Gas pipelines are costly to construct in open terrain and prohibitively costly to construct in populated regions. Much like the railroads, the U.S. pipeline system evolved over many years having been constructed by many competing interests. As such, today's U.S. pipeline system is far from optimized for efficient delivery of sufficient natural gas to meet demand, particularly at peak demand periods. Further, the profile of gas usage by industry, in particular the dramatic growth of gas-fired turbines for electricity production, has exacerbated the daily fluctuation in demand for natural gas beyond the capability of the pipeline system to meet this demand without extraordinary measures. The result has been increasing frequency of price spikes and a fear among commercial users that highly disruptive allocations of natural gas are inevitable in the near future. Moreover, pipelines are highly vulnerable to attack or disruption by other factors such as aging or earthquakes. They are also limited in their ability to transport gas across deep water because of prohibitive cost of construction, naturally occurring clathrate hydrate formation (which can plug the pipeline) and the difficulty of maintaining such structures.
LNG is formed by cooling natural gas below its boiling point, forming a cryogenic liquid that is approximately 600 times denser than atmospheric natural gas. The cryogenic liquid is then transported on and offloaded from very large marine vessels. LNG is by far the most used method of transporting natural gas where pipelines are not possible such as long distance transoceanic transport. Though economic for very large gas reserves (over 5 trillion cubic feet (TCF)) transported over long ocean distances, LNG has a number of drawbacks that have limited its application for shorter distance transport or for small to medium size gas reserves (under 5 TCF). To date, the complexity and cost of LNG production systems has made LNG unsuitable for transportation of natural gas from offshore production facilities. (Offshore production facilities frequently produce large amounts of gas with the oil where the gas cannot be brought to market and must be re-injected or flared). Further, LNG is considered too expensive to be economic for transportation of natural gas for land-based applications where pipelines are not available or sufficient. A further disadvantage of LNG is its inherent volatility, which makes it a potentially attractive target for terrorists. A recent report by Sandia Laboratories concluded that a maritime attack on an LNG tanker could cause widespread and serious damage and injury within a 2,000-foot radius.
High natural gas prices and the widespread existence of smaller “stranded” (i.e. natural gas produced or produce-able but not able to be brought to and sold to market) gas fields, particularly deep water gas fields where pipelines and LNG are impractical, have led to renewed interest in the use of CNG for transport of natural gas. CNG transport and storage systems rely on the use of extremely high pressures to reduce the volume of natural gas by a factor of roughly 175 to 200 times, depending on the particular system. CNG offers a number of advantages over LNG for short to medium distance gas transport. In particular, CNG eliminates the need for the large, fixed infrastructure required to create LNG. A further advantage of CNG is the elimination of complex and expensive degasification terminals on the receiving end of the supply chain. Yet another advantage of CNG is that the bulk of the investment is in the CNG ships themselves making the CNG production/storage units inherently redeploy-able. This redeploy-able nature of CNG systems could also enable the capture of natural gas from smaller land-based gas fields where LNG is not practical. However, CNG has several significant disadvantages. One disadvantage of CNG systems is the reliance on very strong and heavy steel “bottles” to store the pressurized gas. These bottles are limited in diameter and therefore require large numbers of valves and manifolds to control inflow and outflow of gas. A further, and difficult to overstate, disadvantage is the extreme volatility of natural gas at very high pressure. A small leak or failure has the potential to explosively release very large amounts of energy. Yet another disadvantage of CNG systems is their very high cost per constant capacity for storage compared to LNG ships. Another disadvantage of CNG systems is that the proposed CNG ships would be very heavy and are difficult to dry dock for maintenance. A further disadvantage of CNG is the difficulty associated with inspecting the pressure tanks (for cracks or other safety risks) and replacing them when there is a problem.
Increasing demand for clean burning liquid fuels, particularly ultra-low sulfur diesel, has resulted in renewed commercial interest in GTL technologies. GTL predominantly relies on the Fischer Tropsch process for converting natural gas to higher alkanes and liquid fuels. The advantage of GTL is that low density, difficult to transport natural gas can be converted into a dense, energy rich, commercially valuable and easy to transport liquid fuel. GTL production is essentially a refinery process requiring complex systems, machinery and operational skills. A major disadvantage of GTL technology is its complexity and high capital cost. Another disadvantage is the GTL process requires large amounts of expensive catalysts (large amounts because they are difficult to reactivate). A further disadvantage of GTL systems is the high temperatures and pressures required for conversion. While some GTL projects are underway in large reserve locations such as Trinidad and Qatar, the complexity of these systems has, so far, made them impractical for offshore and smaller scale applications. GTL is impractical and non-economic for capture and transport of natural gas from smaller or offshore fields. Furthermore, while GTL is a higher energy density fuel, SNG preserves the natural gas in its original state that continues to be a preferred state for many industries.
The disadvantages of the systems and approaches described above have led to the consideration of natural gas hydrates for natural gas transport. Solid Natural Gas (SNG) offers a number of advantages over LNG, CNG and GTL.
Safety is a significant advantage of SNG versus all other forms of transporting or storing natural gas; the volatility (explosive risk) of SNG is very low. The dissociation of SNG is an endothermic process, meaning that large amounts of heat are required to release the gas stored. The low thermal conductivity and auto cooling effect of hydrate dissociation further reduce the volatility of SNG compared to both LNG and CNG. As compared to LNG and CNG, another strong advantage is the significantly higher temperatures and lower pressures, respectively, required to form and maintain SNG. This directly translates into less complicated containment systems for SNG product versus LNG and CNG. CNG requires an enormous amount of costly and extremely heavy steel to contain the gas at extreme pressures. LNG requires heavily insulated, costly and complex containment systems that can maintain integrity and function at cryogenic temperatures. The formation temperature of SNG is ˜2° C. to 6° C. compared to a liquefaction temperature of −160° C. for LNG. The higher formation temperature translates into a higher Coefficient of Performance for the required refrigeration system, and therefore a higher energy efficiency. A disadvantage of SNG is its lower compression ratio (155:1) compared to both LNG (600:1) and versus CNG (150:1 vs 200:1). The lower effective compression or density factor of SNG translates to a need for larger and/or more ships or containment vessels.